1. Field of the Invention
The invention relates to a method for gas chromatographic analysis of a gas sample, which is conveyed by a carrier gas through a separating device having a downstream thermal conductivity detector, which delivers a chromatogram having peaks for different analytes as a measurement signal.
The invention furthermore relates to a corresponding arrangement for gas chromatographic analysis of a gas sample.
2. Description of the Related Art
In chromatography, a sample of a substance mixture to be analyzed is conveyed by a carrier gas through a chromatographic separating device. Because of different migration rates through the separating device, the analytes i.e., the individual substances of the substance mixture reach the output of the separating device at different times, and are successively detected there by a suitable detector. As its measurement signal, the detector generates a chromatogram that consists of a baseline and a number of peaks corresponding to the separated substances. In practice, the chromatogram is affected by noise, with the individual peaks standing out more or less clearly from the signal noise. With well-resolved peaks, the peak area above the noise-free baseline is proportional to the concentration of the analyte; the peak area, in contrast to the peak height, provides accurate measurement results even for asymmetrical peaks.
In gas chromatography, thermal conductivity detectors are preferably used for detecting the separated analytes with the aid of their substance-typical thermal conductivity. To this end, as described, for example, in EP 1381854 B1, the separated analytes are successively conveyed in a channel past an electrically heated heating filament arranged therein, in which case more or less heat is transferred from the heating filament to the channel wall depending on the thermal conductivity of the analyte flowing past in comparison with that of the carrier gas, and the heating filament is correspondingly cooled or heated to a greater or lesser extent. The electrical resistance of the heating filament therefore changes, and this change in electrical resistance is detected. To this end, the heating filament is conventionally arranged in a measurement bridge that contains another heating filament in a further channel through which a reference gas flows.
The detection sensitivity of the thermal conductivity detector is commensurately greater when the temperature difference between the heating filament and the channel wall is greater, although high temperatures compromise the lifetime of the heating filament. The detection sensitivity also depends on the electrical resistivity of the heating filament, because with a predetermined geometry of the heating filament its total resistance is dictated by this. The greater this total resistance is, the greater is the detection sensitivity. Lastly, chemically aggressive gases may attack and destroy the heating filament.
In the thermal conductivity detector known from EP 1381854 B1, the heating filament consists of gold and/or platinum. At about 15 to 25 Ω, the heating filament resistance achievable with gold is low and limits the detection sensitivity. In order to reach a heating filament resistance typically of 20 Ω, in terms of its dimensions the gold filament must be made very thin (<0.3 μm) and narrow (typically 6 μm) for a length of 1 mm. Such filigree dimensions lead to a very low heat capacity and therefore to a very short response time, but also to poor robustness. Moreover, gases containing hydrogen sulfide can destroy the gold filament. Platinum has a much higher melting temperature than gold and five times its resistivity, with almost the same temperature coefficient of the electrical resistance. The advantage of platinum is its chemical inertness, although this means that production in thin-film technology proves very difficult. Another disadvantage is the catalytic effect of platinum in gas mixtures which contain hydrogen and hydrocarbons.
DE 39 06 405 A1 discloses a thermal conductivity detector for gas analyzers, in which the heating element is not a free-lying heating filament but a resistor layer formed on a support plate. For protection against aggressive gases, the resistor layer is coated with a plasma enhanced chemical vapor deposition (PECVD) layer.
WO 2007/106689 A2, WO 2008/098820 A1 and E. Meng and Y.-C. Tai: “A Parylene MEMS Flow Sensing Array” in Transducers 2003, 2003, Boston, Mass., respectively disclose a thermal mass flow rate sensor in which free-lying heating and/or sensor elements are provided with a protective layer of parylene.
Parylene is a generic term for polymeric coating materials of which, in particular, the parylene types parylene N (poly(para-xylylene)), parylene C (poly(chloro-para-xylylene)), parylene D (poly(dichloro-para-xylylene)) and parylene F (poly(tetrafluoro-para-xylylene)) are industrially employed. Although the melting point of parylene N is very high, at 410° C., the mechanical properties change with an increasing temperature. The thermal stability is the lowest of all the parylene types, particularly in an oxygen environment. Although parylene C has very good barrier properties, i.e., very low gas permeabilities, the melting point at 290° C. is the lowest of the parylene types mentioned here. Parylene D has a relatively high thermal stability of up to 380° C. The mechanical and electrical properties are best preserved under an increase in temperature compared with parylene N and parylene C. At more than 500° C., the melting point of parylene F is far higher than the other three parylene types. Parylene F has the highest melting point, at more than 500° C., compared with the other three parylene types, as well as the highest thermal stability. Parylene F is chemically very similar to parylene N, so that the gas permeability of the two is about the same, i.e., permeable for oxygen and to a very high extent also permeable for hydrogen sulfide.